The science regarding the safety of cannabis use in pregnant teens urgently needs to catch up to the rapidly increasing acceptance, availability, and potency of today?s cannabis, along with increasing co-use of cannabis with tobacco. Pregnant teens may find it more difficult than pregnant adults to quit smoking tobacco and cannabis. Cannabis is three times more potent than the cannabis used in earlier cohort studies of adult mothers. Yet, older studies with adult mothers found subtle yet persistent effects of prenatal cannabis exposure (PCE) and prenatal tobacco exposure (PTE) on behavioral outcomes. This study is based upon the body of evidence showing striking differences between substance use in teen and adult mothers, recent societal changes in cannabis use, and the dangers of co-use with tobacco. It is crucial to determine whether cannabis and co-using pregnant teens differ from other pregnant teens, and to determine the effects of PCE and co-exposure on their infants and family functioning in the postpartum. Aim 1: Determine maternal and family characteristics associated with cannabis use and co-use with tobacco in pregnant adolescents from the 1st trimester through 6 and 12 months postpartum. Aim 2: Identify trimester-specific effects of exposures to PCE and co-use with tobacco on infant neurodevelopment and memory, as well as maternal mental health, perceived stress, and use of social services, controlling for baseline differences among pregnant adolescents. The proposed study uses a mixed-methods approach to investigate cannabis and tobacco use in teen mothers during and after pregnancy, factors associated with their persistence and desistance of cannabis and tobacco use, and the effects of PCE and co-use on mother and infant outcomes at delivery, 6, and 12 months. We will screen 1,120 pregnant teens for 1st trimester substance use, to recruit 620 girls for a prospective, longitudinal study. We will match these 4 groups on race, age, and insurance status: cannabis-only (PCE), tobacco-only (PTE), cannabis + tobacco (PCE+PTE), and non-users. Participants will be seen at the 2nd and 3rd trimesters, and 6 months postpartum. Delivery and neonatal data will be extracted from medical records for all live births, and data on medical and public services usage will be obtained from the electronic health record and the PA Department of Human Services Data Warehouse 12 months postpartum. Maternal substance use and functioning will be measured using Timeline Followback interviews, standardized assessments on tablets, and biological quantification. Infant development, temperament, and secondhand smoke exposure will be assessed 6 months postpartum. For a better understanding of pregnant and parenting teens? beliefs, attitudes, and the social contexts of their cannabis and co-use with tobacco, a subsample (n=15/group) will complete qualitative interviews at each time point. The innovative combination of standardized assessments, rich qualitative data, biological quantification, electronic medical record, and public services data will provide robust and unbiased data of current perinatal cannabis use and co-use with tobacco in pregnant teens, associated maternal characteristics, and infant outcomes.